Add test case for PKCS7 file with zero signers

The test file was created by manually modifying
tests/data_files/pkcs7_data_without_cert_signed.der, using
ASN.1 JavaScript decoder  https://lapo.it/asn1js/

Changes made:
The SignerInfos set was truncated to zero length.
All the parent sequences, sets, etc were then adjusted
for their new reduced length.

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/tests/data_files/pkcs7_data_no_signers.pem b/tests/data_files/pkcs7_data_no_signers.pem
new file mode 100644
index 0000000..b75c991
--- /dev/null
+++ b/tests/data_files/pkcs7_data_no_signers.pem
Binary files differ
diff --git a/tests/suites/test_suite_pkcs7.data b/tests/suites/test_suite_pkcs7.data
index ffeec49..eba0e24 100644
--- a/tests/suites/test_suite_pkcs7.data
+++ b/tests/suites/test_suite_pkcs7.data
@@ -10,6 +10,10 @@
 depends_on:MBEDTLS_SHA256_C
 pkcs7_parse:"data_files/pkcs7_data_without_cert_signed.der":MBEDTLS_PKCS7_SIGNED_DATA
 
+PKCS7 Signed Data Parse with zero signers
+depends_on:MBEDTLS_SHA256_C
+pkcs7_parse:"data_files/pkcs7_data_no_signers.pem":MBEDTLS_PKCS7_SIGNED_DATA
+
 PKCS7 Signed Data Parse Fail with multiple certs #4
 depends_on:MBEDTLS_SHA256_C:MBEDTLS_RSA_C
 pkcs7_parse:"data_files/pkcs7_data_multiple_certs_signed.der":MBEDTLS_ERR_PKCS7_FEATURE_UNAVAILABLE